Field of the Invention
The field of the invention is spectrally broadened laser systems. In particular, the invention relates to short pulse mid-IR lasers with supercontinuum output.
Background of the Invention
Pulsed laser sources, which combine ultra-short or femtosecond (fs) pulse duration, high power, and broad optical spectrum are in great demand for a variety of applications in spectroscopy, sensing, coherence tomography, bio-medical, and so on. Extending the spectrally broad laser oscillations to 2-10 μm wavelengths, the so-called mid-IR range, is highly desired for detection of the substances which feature the resonant fingerprints in the mid-IR fingerprint region, e.g. common molecules (including, but not limited to H2O, CO2, CO, and NH4), biomedical materials, air pollutants, hazardous materials. Moreover, high power mid-IR fs sources with broad and coherent optical spectrum are of extreme importance for the time-resolved studies of the fundamental processes in physics and chemistry.
The standard technique for spectral broadening of fs pulses is so-called supercontinuum generation, SCG. In the past decade SCG has been mostly explored in wavelength scale optical fibers and waveguides. SCG covering the whole mid-IR fingerprint region has been demonstrated very recently in a chalcogenide step-index fiber. However, there are intrinsic peak and average power limitations and alignment sensitivity issues with the methods of fiber- or waveguide-based SCG.
More recently, SCG in bulk materials started to show new opportunities with its advantages such as relative simplicity, flexibility, high peak and average power. See for example, M. Durand, et al., “Study of filamentation threshold in zinc selenide,” Opt. Express 22, 5852-5858 (2014). In bulk materials laser propagation is not restricted by the material cross section profile, so alignment sensitivity is relaxed. Moreover, SCG in some bulk materials features compression of femtosecond input pulse to even shorter output pulse comprising only few optical cycles, see H. Liang, et al., “Three-octave-spanning supercontinuum generation and sub-two-cycle self-compression of mid-infrared filaments in dielectrics,” Opt. Left. 40, 1069-1072 (2015). Both Durant and Liang use an optical parametric amplifier system (OPA) to generate mid-IR.
When a bulk material functions as an optical component, for example transmitting light in a laser system, the bulk material may be referred to as a bulk medium. An important parameter, which defines the ability of the bulk medium to generate a broad continuum, is the so-called critical power for self-focusing, PC. The critical power is a material-specific parameter and is defined by third-order nonlinearity of the material via the nonlinear refractive index n2 where PC is approximated by λ2/n2. The critical power may vary from hundreds of kW (e.g. in II-VI semiconductors like ZnSe and ZnS) to tens of MW (e.g. in YAG or CaF2).
Typically, peak power levels in excess of the critical power cannot be reached directly from a fs oscillator. Therefore, an amplification stage for fs pulses prior to their delivery to a SCG stage is generally employed. Furthermore, many standard fs oscillators (e. g. Ti:Sapphire, fiber-based) emit in the near-IR. Therefore, a stage for frequency conversion of near-IR fs pulses to mid-IR range (e.g. OPA) is used prior to the SCG stage. On the one hand, the techniques for frequency conversion to the mid-IR range and techniques for amplification of fs pulses are well developed. On the other hand, the overall setup with multiple stages is very complex bulky and high-cost, which greatly limits the practical use of existing mid-IR bulk material SCG sources.
A need exists for a pico-second and even more preferentially a femtosecond laser with broad spectral output that does not rely on multiple stages to produce such output. The present inventors have satisfied that need by constructing a laser system with a single stage that amplifies and spectrally broadens in a single pass configuration. Other non-linear effects, such as pulse compression, may be further provided by this laser system.